In the United States the gas distribution industry is increasingly using polyethylene pipe for gas distribution piping. This trend is due to the reduced installation cost, ease of joining the light-weight pipe, and resistance to corrosion exhibited by polyethylene pipe. One method of joining polyethylene piping is to use mechanical fittings. However, mechanical fittings vary in reliability and are very susceptible to errors by workers. Thus, mechanical fittings are generally undesirable in the gas industry.
Numerous patents have issued over the years on various apparatuses and methods for joining polyethylene pipe. One method, illustrated by U.S. Pat. No. 5,125,690 to Taylor, et al., involves the use of various types of heaters embedded in the inner surface of a sleeve which fits over two pipes to be joined. The pipes are inserted into the sleeve in abutment with each other. Upon heating of the embedded wire heater, the regions of the pipe adjacent the sleeve are softened to the point of fusion and the polyethylene pipes and the sleeve are thereby joined together. Essentially, the same approach is employed for saddles and patches, except for the mechanics of holding the pieces in contact prior to completion of the fusion process.
A recurrent problem with methods for joining polyethylene pipes through fusion welding has been uneven or nonuniform heating of the polyethylene elements to be joined. If the heat is not spread evenly over the parts to be joined, voids and/or weak fused regions result at the juncture between the joined parts. These poorly fused regions are weak in tension, shear, and peel.
Fusion welding methods have been developed in the industry which attempt to more evenly distribute the heat over the polyethylene parts being joined. One previous method, illustrated in international PCT patent WO 2/15182 to Ross et al., utilizes heater wire disposed in various configurations depending on the shape of the polyethylene parts being joined. For example, when a saddle fitting is to be attached to a pipe, the wire is wound in a flat or pancake-like circular, elliptical, or rectangular configuration with radial or cross wires in a central open space so as not to interfere with the communication between the saddle and the main pipe. The importance of the shape employed is to get a large surface area covering of the polyethylene. The wire is a ferromagnetic covered copper or like conductive material, as described in U.S. Pat. No. 4,256,945 to Carter et al. The structure that the wire is formed into and the particular type of wire allow for constant temperature regulation and thus uniform heating throughout the polyethylene parts.
Self-regulating heating elements are also known in the art for helping to distribute heat more uniformly. See, for instance, U.S. Pat. No. 4,256,945 to Carter et al., and U.S. Pat. No. 5,125,690 to Taylor et al. In these self-regulating heating elements, the heater is essentially a wire having an inner core of a non-magnetic material, such as copper, which has high thermal and electrical conductivity, and a surface layer of a ferromagnetic material. Wires with this structure function in a temperature self-regulating manner. When a constant current is applied to the wire, the current is substantially confined to the ferromagnetic surface layer until the temperature of the heating element rises to a particular temperature called the Curie temperature. As this Curie temperature is approached, the current migrates or spreads into the non-magnetic core of the heating element wire. As a result of this migration of the current, the resistance of the heating element declines sharply near the Curie temperature so that the power dissipated by the heating element likewise declines. The Curie temperature of the wire is determined by the characteristics of the ferromagnetic surface layer. U.S. Pat. No. 5,125,690 to Taylor et al. utilizes a self-regulating heater which relies on control of the Curie temperature. The heater is embedded in or wrapped around a sleeve placed around the polyethylene parts to be joined. The heater fuses the polyethylene sleeve to the polyethylene parts, thus joining them.
Furthermore, several heating elements consisting of a conductive mesh are known in the industry. However, none appear to be useful in producing uniform heat for fusing together polyethylene pieces. More specifically, U.S. Pat. No. 2,884,509 to Heath teaches a heating element in which some of the strands are conductors and other interwoven strands are resistors. The conductive strands are formed of some suitable metal. The resistors or insulated strands are formed of glass fibers. The strands may be woven together in any suitable weaving pattern. Conductive strands are interwoven and interlocked with respect to each other and with respect to the insulated strands. U.S. Pat. No. 2,065,760 to Smith also teaches a network of interconnected resisting wires and conducting wires used as an electrical heating device.
Although many of the prior art techniques for joining polyethylene pipes through fusion welding have some merit, these techniques are still not optimum in that they still result in uneven or nonuniform heating of the polyethylene elements to be joined to a certain extent. Thus, fused regions tend to be weak in tension, shear, and peel. Accordingly, a need exists in the industry for a better heater and method for providing heat more uniformly across a heat emitting surface so that polyethylene pieces can be optimally fused together.